In electronic circuits with serial transmission interfaces, it is often necessary to filter the input signal to eliminate the parasitic pulses that may result with excessively small pulse widths. If these parasitic pulses are not filtered, they could cause malfunctioning of the electronic circuit. Indeed, the appearance of a pulse could activate a particular operation in the circuit, but the pulse width would not be sufficient to ensure that the operation runs to its complete extent. Furthermore, it is not desirable that a pulse caused by noise on the line should be capable of activating a function of the electronic circuit. This is why a filtering device is usually provided. This filtering device may include an RC lowpass filter.
The use of an RC lowpass filter of this kind nevertheless has a few drawbacks. The resistance and capacitance values of the RC filter may vary from one electronic circuit to another, owing to variations inherent to the method of manufacture used. These values of resistance and capacitance will also vary with the level of the ambient temperature at which the electronic circuit functions.
The cutoff value of the RC filter depends directly on the amplitude of the pulse. This is shown in FIGS. 1a to 1c in the appended drawings. In these figures, the input signal is referenced Sin and the filtered output signal is referenced Sf. FIG. 1a shows the charging and discharging at the terminals of the capacitor when a pulse I with an amplitude V appears on the input signal Sin (FIG. 1b). The cutoff value d1 of the RC lowpass filter is given by V/2. The filtered signal Sf (FIG. 1c) at the output of the filter has a pulse shortened by the duration d1 corresponding to the time taken to charge the capacitor from zero to V/2.
The filtered signal thus has a shortened pulse duration d2 in the figure. A shortening of this kind of the pulse width is highly inconvenient in practice. Indeed, the system may find itself with a filtered pulse width d2 smaller than the cutoff value d1. For example, if there is an RC filter to filter the pulse widths smaller than 300 ns (d1=300 ns) and if the input pulse has a width of 310 ns, the filtered pulse will have no more than a width d2 equal to 10 ns, leading to an erratic operation of the electronic circuit. With a filtering system of this kind, it is therefore necessary to set a specification for the user requiring that pulses be presented with a width d far greater than the cutoff value d1 of the filter, to take account of the zone of erratic operation.
If d1 is the cutoff value equal to the minimum width of a pulse for accurate operation of the electronic circuit, the specification set a minimum width dmin=2.multidot.d1. The three operating zones are then the following, for an input pulse with a width d:
d&lt;d1: completely filtered pulse, PA1 d1&lt;d&lt;2.multidot.d1: zone of erratic operation, and PA1 d&gt;2.multidot.d1: zone of accurate operation.
Finally, it has been seen that the cutoff value d1 of the filter depends on the amplitude V of the pulse. In one transmission system using the variation of the amplitude of the pulse as an information transmission system, a filtering operation of this kind is therefore not applicable since it would dictate cutoff values that differ according to the amplitude of the pulse received.